1. Field of the Invention
The invention relates to a vehicle inner panel that is underneath and overlaps with an outer panel on the outside of a hood of a vehicle or the like. More particularly, the invention relates to a vehicle inner panel for improved pedestrian protection with respect to the head of a pedestrian in a frontal collision.
2. Description of the Related Art
In a vehicle provided with a hood on the front portion of the vehicle body, the vehicle hood is made of an outer panel, an inner panel, and other reinforcing members and the like. In order to ensure the rigidity and strength of the hood itself, the outer panel and the inner panel are typically joined so as to form a structure with a closed cross-section and a space in between the two panels.
Japanese Patent Application Publication No. 2001-151159 (JP-A-2001-151159) and Japanese Patent Application Publication No. 2003-205866 (JP-A-2003-205866) describe a structure of a related roof which ensures dent resistance and tensile strength required for hoods while increasing pedestrian protection performance during a frontal collision by forming a plurality of beads having U-shaped cross-sections generally parallel with each other near the center of the inner panel.
Currently, a HIC (Head Injury Criteria) value calculated from an Acceleration—Time waveform at impact is typically used to evaluate pedestrian protection performance during a frontal collision. A lower HIC value indicates a lower risk of injury to the head of a pedestrian during a collision. Therefore, as described above, a hood structure with a low HIC value is desirable after ensuring dent resistance and tensile strength.
The acceleration waveform with respect to the head when the head collides with an area of the hood near the center is such that a first wave is generated when the head contacts the hood at the onset of the collision, and then a second wave is generated when the hood portion that has been pressed against by the head moves downward and contacts internal parts such as the engine arranged underneath the hood.
The HIC value can be effectively lowered by reducing the second acceleration wave in particular. Therefore, during head impact, the hood needs to absorb the impact energy before it contacts the internal parts. Thus it is desirable to have sufficient space between the hood panel and the internal parts.
However, when considering the fact that parts necessary for a vehicle are housed within a limited space under the hood, it is often difficult to ensure sufficient space between the hood panel and the internal parts.
Therefore, it is preferable to make a hood structure that either reduces the deformation stroke by making the first acceleration wave during a head impact as large as possible and increase the amount of energy absorbed at the onset of the collision, or reduces the second acceleration wave generated when the hood contacts the internal parts.
With the foregoing hood inner structure in which a plurality of beads having U-shaped cross-sections are formed parallel with one another, the beads increase the flexural rigidity of the hood panel, thereby suppressing bending deformation during impact with the head of a pedestrian so the impact load can be dispersed. Therefore, the area displaced by the head impact, and thus the weight, increases compared with when the beads are not provided, so the first acceleration wave during a head impact increases. Also, because the beads are formed generally parallel with one another, the hood more easily deforms when it contacts the internal parts, thereby reducing the second acceleration wave.
However, with the requirements with respect to pedestrian protection becoming increasingly stringent, further improvements in pedestrian protection performance are desired.
Therefore, to further improve pedestrian protection performance, Japanese Patent Application Publication No. 2006-44542 (JP-A-2006-44542) proposes a hood inner structure in which the beads having U-shaped cross-sections are provided as described above, wherein the height of the U-shaped cross-section is changed depending on the portion taking into account actual impact conditions, and Japanese Patent Application Publication No. 2006-44543 (JP-A-2006-44543) a hood inner structure in which the beads having U-shaped cross-sections are provided as described above, wherein a flat part is provided midway in the vertical walls of the U-shaped cross-sections.
However, in such U-shaped cross-section bead structures, in order to further increase the first acceleration wave, it is necessary to disperse the stress not only in the direction along the beads, but also in a direction orthogonal to the beads. Further, it is desirable to increase the deformation rigidity of the beads so that localized deformation does not occur during a head impact from above the vehicle. At the same time, in order to reduce the second acceleration wave, it is desirable to have an easily deformable structure with a low deformation load with respect to a load applied from below the vehicle upon contact with internal parts. However, no related art has been able to meet these needs.